1. Field of the Invention
The present invention relates to a color measurement method of determining colorimetric values of a measurement object color area on a printed form, typically color patches constituting a color chart on the printed form.
2. Description of the Related Art
Hitherto, it is performed that an input device, for example, a color scanner or an electronic still camera, is used to take an image (hereinafter, it is referred to as an original image) in to obtain image data, so that an reproduced image of the original image is obtained in accordance with the image data by a printing machine or a printer. In this case, there are determined a color reproduction characteristic (profile) associating a color on the original image with a color on the image data, according to the input device, and a color reproduction characteristic (profile) associating a color on the image data with a color on the reproduced image, according to the output device such as a printing machine and a printer, and image data obtained from the original image by the input device is converted into image data suitable for the output device in accordance with both the color reproduction characteristics, so that the reproduced image is outputted in accordance with the image data for the output device. This feature makes it possible to obtain the reproduced image coincident with the original image in color.
The same matter as the above will occur between output devices. Next, there will be explained an example.
Hitherto, when a printing machine is used to perform a color image printing, it is performed that prior to the printing, a color printer and the like is used to create a proof image which is intended to be similar in color to an image to be printed by the printing machine. When the printer creates the proof image, detected are a color reproduction characteristic (a printing profile) describing a relationship between image data and a color of the actual printed matter, associated with a printing machine of interest, and a color reproduction characteristic (a printer profile) describing a relationship between image data and a color of an image to be actually printed, associated with a printer. Image data for printing is converted into image data for printers in accordance with the printing profile and the printer profile, so that a proof image is created in accordance with the image data for printers thus converted. Thus, it is possible to create a proof image that is coincident with the actual printed matter in color.
In order to obtain a proof image that is coincident with the actual printed matter in color in the manner as mentioned above, there is a need to determine with great accuracy color reproduction characteristics (profiles) of the input device and the output device. When the color reproduction characteristics (profiles) are determined, for example, in case of the input device, a color chart, in which color patches are arranged, is read by the input device and then converted into image data, and coordinates (CYMK values, or RGB values) of color spaces (a device color space: for example, a CMYK color space consisting of four colors of cyan (C), magenta (M), yellow (Y) and black (K), or an RGB color space consisting of three colors of red (R), green (G) and blue (B))) on the image data are determined. And the same color chart is measure by a spectrocolorimeter to determine coordinates (L*a*b* values or XYZ values) of a colorimetry color space (for example, L*a*b* color spaces or XYZ color spaces), and the coordinates on the device color space are associated with the coordinates on the colorimetry color space. Thus, it is possible to determine the color reproduction characteristics (profile) of the input device.
When the color reproduction characteristics (profile) of the output device is determined, image data, which corresponds to a color chart wherein color patches are arranged, is created, the output device outputs the color chart in accordance with the image data thus created, the color chart is measured by the spectrocolorimeter, and the coordinates of the color space (the device color space) on the image data are associated with the coordinates of the colorimetry color space. Thus, it is possible to determine the color reproduction characteristics (profile) of the output device.
By the way, a printed form, which includes fluorescent brightening agent to enhance white so that an image looks more beautiful, comes onto the market. In the event that an original image is recorded on a printed form including the fluorescent brightening agent, or in the event that a printing image or a print image is outputted onto the printed form including the fluorescent brightening agent, when a measurement light source used in measurement for colorimetric values of color charts when a profile is produced is different from an observation light source for illuminating images such as an original image, a printing image and a print image when those images are observed, a difference between the measurement light source and the observation light source in emission quantity of the fluorescent brightening agent included in the printed form causes discrepancy in spectral reflectance. Thus, it happens that although there is obtained coincidence in value derived through a color measurement, there is obtained no association in color through the visual observation.
As a method of solving this problem, there is proposed a technology (cf. Japanese Patent Laid Open Gazette Hei. 10-176953) that at the time of color measurement, the color measurement is performed on each of color patches constituting a color chart in two ways according to one of which light of a shortwave is cut and another light of a shortwave is not cut, so that emission quantity of the fluorescent brightening agent is determined and the colorimetric value is correct to meet the observation light source. This method is expressed by formulas as follows.
X=X0+Eil*(X1xe2x88x92X0) 
Y=Y0+Eil*(Y1xe2x88x92Y0) 
Z=Z0+Eil*(Z1xe2x88x92Z0)xe2x80x83xe2x80x83(1) 
Where X0, Y0 and Z0 denote XYZ values measured using a measurement light source wherein a short wavelength side is cut, and X1, Y1 and Z1 denote XYZ values measured using a measurement light source wherein a short wavelength side is not cut. Eil denotes an index indicative of the influence by a fluorescent brightening agent, and is expressed by Eil=ES/EL, where EL denotes fluorescence excitation intensity on the measurement light source wherein a short wavelength side is not cut, and E5 denotes fluorescence excitation intensity on the observation light source. This Eil is a value, which can be determined beforehand taking the observation light source and the like into consideration.
According to adoption of the above-mentioned former method, it is possible to correct XYZ values obtained through a color measurement to XYZ values obtained through an observation light source.
However, according to the above-mentioned former method, it is necessary to perform twice measurements on each of a plurality of color patches constituting a color chart using the measurement light source wherein a short wavelength side is cut and the measurement light source wherein a short wavelength side is not cut. It is usual that a color chart is constituted of hundreds of color patches arranged. It would take a long time to perform twice measurements on each of such a many color patches, and it is a problem in workability.
In view of the foregoing, it is an object of the present invention to provide a color measurement method capable of determining colorimetric values corrected in the influence of the fluorescent brightening agent, which are involved in the actual observation, with excellent workability.
To achieve the above-mentioned object, the present invention provides a color measurement method of determining colorimetric values on a colorimetric object color area on a printed form, said color measurement method comprising:
a reference correction quantity obtaining step of determining a reference correction quantity, which is a reference when the colorimetric values on the colorimetric object color area are corrected, the reference correction quantity being associated with a difference between two colorimetric values obtained when a first light and a second light wherein light of a predetermined short wavelength area included in the first light is cut are used to measure a reference color area on the printed form;
a color measurement step of obtaining a first colorimetric value by color measuring the colorimetric object color area using one of the first light and the second light;
a weight obtaining step of measuring a spectral reflection characteristic of at least predetermined short wavelength area of the colorimetric object color area using said one light to determine a weight for correction involved in correction of said first colorimetric value in accordance with said spectral reflection characteristic; and
a correction step of correcting said first colorimetric value obtained in said color measurement step in accordance with the reference correction quantity obtained in said reference correction quantity obtaining step and the weight obtained in said weight obtaining step to determine a second colorimetric value.
According to the color measurement method of the present invention, two light sources are used to measure only a reference color area on a printed form so that a reference correction quantity is determined. And with respect to another area (for example, hundreds of color patches), weight and colorimetric value are determined and the colorimetric value is corrected in accordance with the weight and the reference correction quantity. In the color measurement step the colorimetric value is measured, and in the weight obtaining step the spectral reflection characteristic is measured. When the colorimetric value is measured, as a premise, the spectral reflectance is measured. Usually, a colorimeter outputs both the colorimetric value and the spectral reflectance. Thus, in the color measurement step and the weight obtaining step, it is sufficient for both the steps to perform the measurement once. Or alternatively, it is acceptable that only the spectral reflectance is measured and the colorimetric value is determined in the arithmetic unit for performing the correction operation. The xe2x80x9ccolor measurement stepxe2x80x9d referred to in the present invention includes an aspect of determining the colorimetric value through such a correction operation.
In the color measurement method according to the present invention as mentioned above, it is acceptable that said reference correction quantity obtaining step measures the reference color area on the printed form or another printed form having same characteristics as the former printed form using the first light and the second light so that a third colorimetric value and a fourth colorimetric value are obtained, and determines a value representative of a difference between the third colorimetric value and the fourth colorimetric value, said value being associated with said reference correction quantity.
Simply, in this manner it is possible to determine the reference correction quantity.
Further, in the color measurement method according to the present invention as mentioned above, it is preferable that said reference correction quantity obtaining step comprises:
a first step in which the first light and the second light are used to measure respective reference color areas of a plurality of printed forms having mutually different characteristics, so that a value representative of a difference between a third colorimetric value involved in the first light and a fourth colorimetric value involved in the second light is determined on each of the plurality of printed forms, and said one light adopted in said color measurement step is used to measure a spectral reflection characteristic of at least predetermined short wavelength of the respective reference color areas of the plurality of printed forms, and an index value associated with the value representative of the difference is determined in accordance with the spectral reflection characteristic, so that an association definition representative of an association between the value representative of the difference and the index value; and
a second step in which said one light adopted in said color measurement step is used to measure a spectral reflection characteristic of said predetermined short wavelength of the reference color area on the printed form having the colorimetric object color area or another printed form having same characteristics as the former printed form, and the index value according to the spectral reflection characteristic is determined, and the index value thus determined is converted into the value representative of said difference referring to the association definition obtained in said first step so that said reference correction quantity is determined.
According to this feature of the present invention as mentioned above, at only the step of determining the association definition, there is a need to use both the light (the second light) in which the light at the short wavelength side is cut and the light (the first light). However, it is sufficient that this is performed once in a maker side for example, at a preparation step, and in the actual measurement work, it is effective that either one of those two lights is used.
An advantage that the reference correction quantity obtaining step is arranged in the manner as mentioned above resides in the point that as mentioned above, the use of two types of light is performed once in a maker side for example and it is distributed to a number of users. In this case, the reference correction quantity obtaining step paying attention to users can be expressed as follows. That is, it is preferable that said reference correction quantity obtaining step is a step in which said one light adopted in said color measurement step is used to measure a spectral reflection characteristic of at least predetermined short wavelength of the reference color area on the printed form having the colorimetric object color area or another printed form having same characteristics as the former printed form, and the index value according to the spectral reflection characteristic is determined, and
the index value thus determined is converted into the reference correction quantity referring to an association definition representative of an association between the index value and the reference correction quantity, said association definition being obtained beforehand.
In this case, it is effective for the measurement that there is prepared only a light source fixed on either one of a light source in which a short wavelength is cut and a light source in which a short wavelength is not cut.
In the color measurement method according to the present invention a mentioned above, it is preferable that said reference correction quantity obtaining step is a step of obtaining the reference correction quantity associated with a difference between two colorimetric values obtained when a white area on the printed form having the colorimetric object color area or another printed form having same characteristics as the former printed form.
According to the present invention, it is permitted to adopt, as the reference color area, for example, a predetermined density of gray of area or the like. However, it is possible to adopt as the reference color area a white area on the printed form, that is, the printed form per se, without preparing such a special area.
In the color measurement method according to the present invention as mentioned above, it is preferable that said weight obtaining step is a step in which a spectral reflectance of at least part of wavelength area of wavelength areas of 500 nm or less is measured and the weight is determined in accordance with the spectral reflectance.
The reason why this is to do so is that the influence of the fluorescent brightening agent appears at areas of 500 nm or less.
In the color measurement method according to the present invention as mentioned above, it is preferable that said weight obtaining step is a step in which the weight is determined in accordance with a product of a predetermined short wavelength area of the spectral reflection characteristic of the colorimetric object color area, said spectral reflection characteristic being obtained through measurement. In this case, it is preferable that said weight obtaining step is a step in which the weight is determined in accordance with a function including a square of said product.
As will be described later, an adoption of such an arithmetic operation makes it possible to suitably determine the weight.
Of the color measurement method according to the present invention as mentioned above, when there is adopted a method in which the above-mentioned association definition is used in the reference correction quantity obtaining step, it is preferable that said reference correction quantity obtaining step is a step in which a spectral reflection characteristic of a predetermined wavelength of 500 nm or less is measured and said index value is determined in accordance with the spectral reflection characteristic. Or alternatively, it is preferable that said reference correction quantity obtaining step is a step in which spectral reflection characteristics of two predetermined wavelengths are measured and a value representative of a difference between the spectral reflection characteristics is determined in form of the index value.
The reason why the index value is determined in accordance with the spectral reflection characteristic of wavelength of 500 nm or less is that the influence of the fluorescent brightening agent appears at areas of 500 nm or less. The reason why a difference between the spectral reflection characteristics is determined in form of the index value is that the difference, for example, a difference between two waves in spectral reflectance or a slant is preferable as the index value.